High speed noise detection and reduction in active pixel sensor arrays

ABSTRACT

A system for detecting high speed noise in active pixel sensors includes a photodiode for receiving low levels of light, a reset transistor, an amplifier transistor, a row select transistor, and a high-speed analog-to-digital converter. The reset transistor gate receives a reset signal, and the reset transistor drain receives a reset voltage. The amplifier transistor gate is connected to the photodiode and the reset transistor&#39;s source. The amplifier transistor receives a supply voltage at the drain terminal. The row select transistor gate terminal receives a row select signal. The row select drain terminal is connected to the amplifier transistor source terminal. The high-speed analog-to-digital converter includes an analog input port connected to the row select transistor source and a digital output port capable of resolving high-speed excitation events received by the photodiode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/953,761, filed on Dec. 10, 2007, and entitled “NOISE REDUCTION INACTIVE PIXEL SENSOR ARRAYS”, which is incorporated herein by referencein its entirety.

COPYRIGHT & TRADEMARK NOTICES

A portion of the disclosure of this patent document contains material,which is subject to copyright protection. The owner has no objection tothe facsimile reproduction by any one of the patent document or thepatent disclosure, as it appears in the Patent and Trademark Officepatent file or records, but otherwise reserves all copyrightswhatsoever.

Certain marks referenced herein may be common law or registeredtrademarks of third parties affiliated or unaffiliated with theapplicant or the assignee. Use of these marks is for providing anenabling disclosure by way of example and shall not be construed tolimit the scope of this invention to material associated with suchmarks.

TECHNICAL FIELD

The present invention relates generally to imaging sensors and, moreparticularly, to noise reduction and data recovery in imaging activepixel sensors.

BACKGROUND

An image sensor is a device that converts a visual image to an electricsignal. The image sensor is used chiefly in digital cameras, but mayalso be found in other imaging devices. The sensor is usually anintegrated circuit containing an array of charge-coupled devices (CCDs)or CMOS pixel sensors, where the latter are referred to as active pixelsensors (APS).

The quality of an image produced using an image sensor may be distorteddue to various noise levels introduced during capturing and producingthe image. For example, as the area per active pixel element for imagingsensors shrinks, the pixel elements gather less light and may becomemore susceptible to current fluctuation (i.e., dark noise) generated bybackground radiation in a pixel element. The longer the pixel elementsare exposed, in low light conditions, and the higher the amplificationgain per pixel element, the more dark noise will result and cause imagedistortion.

Noise from random excitations of CCD or APS pixel elements may occurnon-linearly over time, such as in single uncorrelated spikes or burstsof short duration. Noise from random excitations may come from theincreased sensitivity of the pixels and high-energy random photons beingdetected in regions (e.g., the near infrared (NIR) or extremeultraviolet (EUV)) that are not intended to be detected. For example,alpha particles or gamma ray photons can cause excitation in individualpixels. Such noise sources are high-speed, random, individual eventsthat may typically occur in a single pixel, and account for a largefraction of total energy absorbed by the pixel in selected interval oftime (i.e., frame), rather than a gradual, cumulative event.

Currently, in active pixel sensors that are controlled by row andcolumn, it is not possible to detect a random excitation which depositsan excessive amount of energy in the pixel. To remedy this problem, acorrection function is usually utilized which requires interpolation, orcolor channel filtering. Unfortunately, however, the data from cellsthat underwent excitation may be permanently lost, as a result ofapplying the correction function, all to the detriment of image quality.

There is a need, therefore, for methods and system to save the data thatmay be lost due to the above correction function to maintain a highpicture quality, especially in a high-sensitivity sensor that may besubjected to high dose excitations of very short intervals.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are understood by referring to thefigures in the attached drawings, as provided below.

FIG. 1 illustrates an exemplary active pixel sensor circuit, inaccordance with one embodiment.

FIG. 2 is an active pixel sensor cell circuit for sampling detectorresponse, in accordance with one embodiment.

FIG. 3 is a graph illustrating an exemplary amplitude response time ofthe circuit of FIG. 1 to nonlinear excitation events.

FIG. 4 is an active pixel sensor cell circuit for detecting a fastnonlinear excitation and correcting the cell output, in accordance withone embodiment.

FIG. 5 is an active pixel sensor cell circuit for detecting a fastnonlinear excitation and correcting the cell output, in accordance withanother embodiment.

FIG. 6 is an active pixel sensor cell circuit for detecting a fastnonlinear excitation and correcting the cell output, in accordance withyet another embodiment.

Features, elements, and aspects of the invention that are referenced bythe same numerals in different figures represent the same, equivalent,or similar features, elements, or aspects, in accordance with one ormore embodiments.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

For purposes of summarizing, certain aspects, advantages, and novelfeatures of the invention have been described herein. It is to beunderstood that not all such advantages may be achieved in accordancewith any one particular embodiment of the invention. Thus, the inventionmay be embodied or carried out in a manner that achieves or optimizesone advantage or group of advantages without achieving all advantages asmay be taught or suggested herein. A system for detecting high-speednoise in active pixel sensors includes a junction photodiode and a resettransistor. The reset transistor has a first source terminal, a firstgate terminal and a first drain terminal, wherein the first sourceterminal is connected to the junction photodiode. The first gateterminal receives a reset signal, and the first drain terminal receivesa reset voltage.

The system further includes an amplifier transistor having a secondsource terminal, a second gate terminal and a second drain terminal. Thesecond gate terminal is connected to the junction photodiode and thereset transistor's first source terminal. The amplifier transistor'ssecond drain terminal receives a supply voltage. The system furtherincludes a row select transistor having a third source terminal, a thirdgate terminal and a third drain terminal.

The third gate terminal receives a row select signal, and the thirddrain is connected to the second source terminal of the amplifiertransistor. A high-speed analog-to-digital converter having a firstanalog input port and a first digital output port. The first analoginput port is connected to the third source terminal of the row selecttransistor.

A method for detecting high-speed noise in active pixel sensorscomprises receiving a light signal at a junction photodiode in presenceof a high speed excitation event. The received light signal is convertedto an amplified voltage, and the amplified voltage is converted to adigital signal, wherein the converting to a digital signal isaccomplished using an analog-to-digital converter circuit capable ofresolving event signals up to a first frequency threshold.

One or more of the above-disclosed embodiments in addition to certainalternatives are provided in further detail below with reference to theattached figures. The invention is not, however, limited to anyparticular embodiment disclosed.

An active pixel sensor array may comprise individual active pixel sensorcells arranged in rows and columns. In a CMOS image sensor chip, theactive pixel sensor includes extra circuitry associated with each photosensor which converts the light energy to a voltage. Each pixel sensorcontains a photodetector which is connected to an active transistorreset and readout circuit. Additional circuitry on the chip converts thevoltage to digital data.

As shown in FIG. 1, a CMOS APS 100 includes three transistors as well asa photodetector. The photodetector may usually be a junction photodiode.Light causes a current, or integration of charge on the ‘parasitic’capacitance of the junction photodiode 110, creating a voltage changerelated to the incident light. A reset transistor 120 acts as a switchto reset the device. When reset transistor 120 is turned on, thephotodiode is effectively connected to the power supply, VRESET,charging the junction to this voltage.

An amplifier transistor 130, in one embodiment, acts as a bufferamplifier which allows the pixel voltage to be observed without removingthe accumulated charge. Amplifier transistor's 130 power supply, VDD,may be coupled to the power supply of the reset transistor 120, but maybe associated with a separate voltage level.

In accordance with one embodiment, FIG. 2 shows an individual threetransistor active pixel sensor (APS) cell 200, which includes a p-njunction photodiode 110 with an anode connected to ground. The p-njunction is an effective capacitance coupled to the source of a resettransistor 120 and a gate of an amplifying transistor 130 that serves asa source follower amplifier. Amplifying transistor 130 senses the chargefrom junction photodiode 110 without draining the junction photodiode100, and provides a voltage output as a measure of the charge. Thedrains of the reset transistor 120 and of the amplifying transistor 130may be connected to a supply voltage V_(DD). In the exemplary embodimentshown in FIG. 2, the reset transistor 120 drain may be connected to adifferent voltage V_(RESET).

The amplifying transistor 130 is coupled to a high-speedanalog-to-digital converter (ADC) 150 through a row select accesstransistor 140, for example. The speed of the ADC 150, and similardigital components provided in more detail below, may be selected to befast enough to detect sudden excitation changes in voltage levels up toa specified frequency response. When activated, reset transistor 120places a charge on the junction of the photodiode 110 connected to thepower supply V_(RESET). This charge is drained through the junctionphotodiode 110 at a rate proportional to the intensity of light incidenton junction photodiode 110. A signal proportional to this intensity canbe read from the APS to a data line through ADC 150 by enabling rowselect access transistor 140 when a row select signal is applied, forexample.

In a first operation state (e.g., a normal course of operation),V_(RESET) may be the same for one or more (e.g., all) APS cells. Thus,at the beginning of an imaging cycle, one or more junction photodiodes110 may be charged to V_(RESET) when the reset signal is set. As photonsarriving at junction photodiode 110 cause charges to drain to ground,the voltage appearing at the gate of amplifier transistor 130 may changefrom V_(RESET) to ground, for example, resulting in a change in voltageseen at ADC 150. The voltage at ADC 150 may start, for example, atapproximately zero and may change, depending on light intensity, to avalue up to a saturation maximum equal approximately to V_(DD).

In the first operation state, the shutter speed (i.e., the length of thetime frame between resending of reset signals), may be set for theexpected light conditions so that the photo intensity is not sufficientto drive the voltage at the gate of the amplifier transistor 130 toground. Should this happen, ADC 150 may receive the full value of thesupply voltage (i.e., V_(DD)) which corresponds to saturation. Thus, inaccordance with one embodiment, ADC 150 provides an output that iswithin the dynamic range set by V_(DD).

A sudden non-linear and discontinuous (i.e., quantum) excitation maycause a sudden drop in the charge left to drain through photodiode 110junction, with the result that ADC 150 may register a saturation voltagebefore the end of the time frame, or at least register an output greaterthan expected in the absence of the excitation. In one embodiment, ADC150 may have a sampling speed that is capable of sampling the voltagerise quickly enough to resolve the occurrence and magnitude of anunexpected discontinuous or quantum shift.

FIG. 3 is a graph of an exemplary response time to a nonlinearexcitation event for the APS cell circuit 100 illustrated in FIG. 2. Asshown, in a 1/60th second (i.e., 0.0167 sec) time frame, ADC 150registers a slow increase in accumulated photons, for example, in a lowlight level condition that may not saturate the range defined by V_(DD).An exemplary non-image related excitation is shown occurring atapproximately 0.008 seconds, which results in a discontinuous jump inthe sampled signal at ADC 150, which has a frequency response highenough to resolve the excitation event at a satisfactory speed. As aresult of the excitation event, the accumulated signal may reachsaturation in ADC 150 before the end of the time frame.

The compensatory shift provided to the input of ADC 150 may be used tocorrect for a sudden offset signal in a variety of ways. For example, inAPC cell circuit 400, shown in FIG. 4, the magnitude of the shift may bedetected as a high-speed transient spike through a capacitor 160 betweenthe source terminal of amplifier transistor 130 and driver circuit 170.Driver circuit 170 may provide a partial reset signal to the gate ofreset transistor 120 sufficient to reset, as needed, the charge atjunction photodiode 110 as seen at the gate of amplifier transistor 130.

The charge reset value desirably may be equal to a charge that wouldhave been present in the absence of the excitation, thus providing avoltage at the gate of amplifier transistor 130 that would have beenapproximately equal to the same value of voltage that would have beenpresent in the absence of the excitation, and provide continuity ofimaging behavior. Continued photon absorption results in the chargedraining through photodiode 110 and voltage at the gate of resettransistor 120 then changing correspondingly from the reset value as ifthe excitation had not occurred.

In an alternative embodiment (not shown), capacitor 160 may instead becoupled between row select transistor 140 and ADC 150 to driver circuit170, wherein driver circuit 170 provides the same correction required asin the embodiment shown in FIG. 4. The term coupled as used hereinrefers to a physical or logical connection between electrical componentsof the system.

Alternatively, in another embodiment (not shown), when an event isdetected, the maximum pre-excitation voltage received at ADC 150 may beextrapolated to the end of the time frame ( 1/60^(th) second, forexample) to fill the pixel location to a non-saturated level that wouldhave taken place in the absence of the excitation. That is, thesaturation value output by ADC may be replaced by the extrapolatedvalue. Referring to FIG. 3, it may be seen that the slope of the signalchanges before and after the event may be substantially the same. Themagnitude of the discontinuous increase in ADC 150 voltage measured maybe subtracted, using software or hardware, and the signal reset to theextrapolated voltage.

FIG. 5 shows another exemplary embodiment of an APS cell 500, in whichthe sudden change in voltage seen at ADC 150 may be used to generate areset signal on the reset transistor 120 gate and desirablysimultaneously adjust V_(RESET) to charge junction photodiode 110 to,for example, the same or an extrapolated voltage that would be seen atthe gate of amplifier transistor 130 in the absence of the excitation.In imaging processes where the normal rate of change of charge drainageat photodiode 110 may be assumed to be constant over the interval of thetime frame, a high-speed predictor circuit 510 operably coupled to ADC150 may predict the voltage that may be expected in the next samplingperiod of ADC 150.

A sudden measured change that deviates from the predicted value may beinput to a high-speed DAC 520 to provide a variable analog reset to thevalue of V_(RESET) at the drain of reset transistor 120. Predictorcircuit 510 may supply a reset signal to the gate of reset transistor120 to permit charging of the junction of photodiode 110 to a levelcorresponding to the voltage predicted at the input to ADC 150. In acertain embodiment, predictor circuit 510 and DAC 520 may have frequencyresponses substantially the same as ADC 150 in order to respond toexcitation events as needed.

FIG. 6 illustrates an APS cell 600, in accordance with yet anotherembodiment, wherein a predictor 510 is calibrated to instruct DAC 620 toadjust the supply voltage V_(DD) to the drain of amplifier transistor130 to reduce the voltage appropriately to match the voltage just priorto the occurrence of the nonlinear excitation event. Predictor 610 maybe adapted to instruct DAC 620 to continuously adjust the supply voltageV_(DD) for the duration of the time frame of light accumulation (e.g.,1/60^(th) of a second). In doing so, predictor 610 may preventsaturation of the pixel output signal.

It may be appreciated that, while the current drain through junctionphotodiode 110 may, for example, be substantially linear (e.g., undernormal operations) over the length of the time frame, the rate of changeof V_(DD) provided by DAC 620 may have to obey a nonlinear timedependency to insure that ADC 150 receives a signal that mimics a normalrate of photon arrival at junction photodiode 110. Predictor 610 maycontrol this relationship through DAC 620, and may distinguish betweenthe normally linear change produced at the input of ADC 150 and anysudden nonlinear changes arising from external excitations.

Embodiments described above illustrate but do not limit the invention.Therefore, it should be understood that the invention can be practicedwith modification and alteration within the spirit and scope of theappended claims. The description is not intended to be exhaustive or tolimit the invention to the precise form disclosed. These and variousother adaptations and combinations of the embodiments disclosed arewithin the scope of the invention and are further defined by the claimsand their full scope of equivalents.

1. A system for detecting high-speed noise in active pixel sensors,comprising: a junction photodiode; a reset transistor having a firstsource terminal, a first gate terminal and a first drain terminal,wherein the first source terminal is connected to the junctionphotodiode, the first gate terminal receives a reset signal, and thefirst drain terminal receives a reset voltage; an amplifier transistorhaving a second source terminal, a second gate terminal and a seconddrain terminal, wherein the second gate terminal is connected to thejunction photodiode and the reset transistor's first source terminal,and wherein the amplifier transistor's second drain terminal receives asupply voltage; a row select transistor having a third source terminal,a third gate terminal and a third drain terminal, wherein the third gateterminal receives a row select signal, and the third drain is connectedto the second source terminal of the amplifier transistor; and adigital-to-analog converter having a third digital input port and athird analog output port, wherein the third analog output port of thedigital-to-analog converter is connected to the second drain of theamplifier transistor.
 2. The system of claim 1, further comprising: ananalog-to-digital converter having a first analog input port and a firstdigital output port.
 3. The system of claim 2, wherein the first analoginput port is connected to the third source terminal of the row selecttransistor.
 4. The system of claim 2, wherein the first digital outputport is connected to a predictor circuit having a second digital inputport.
 5. The system of claim 2, wherein the analog-to-digital converteris a high-speed analog to digital converter.
 6. The system of claim 1,further comprising: a predictor circuit having a second digital inputport, a second digital output port and an a second reset signal outputport.
 7. The system of claim 6, wherein the second digital input port ofthe predictor circuit is connected to a first digital output port of ananalog-to-digital converter.
 8. The system of claim 6, wherein thesecond reset signal output port of the predictor circuit is connected tothe first gate terminal of the reset transistor.
 9. The system of claim6, wherein the second digital output port of the predictor circuit isconnected to a third digital input port of a digital-to-analogconverter.
 10. The system of claim 1, wherein an anode of the junctionphotodiode is connected to ground.
 11. A system for detecting high-speednoise in active pixel sensors, comprising: a junction photodiode; areset transistor having a first source terminal, a first gate terminaland a first drain terminal, wherein the first source terminal isconnected to the junction photodiode, the first gate terminal receives areset signal, and the first drain terminal receives a reset voltage; anamplifier transistor having a second source terminal, a second gateterminal and a second drain terminal, wherein the second gate terminalis connected to the junction photodiode and the reset transistor's firstsource terminal, and wherein the amplifier transistor's second drainterminal receives a supply voltage; a row select transistor having athird source terminal, a third gate terminal and a third drain terminal,wherein the third gate terminal receives a row select signal, and thethird drain is connected to the second source terminal of the amplifiertransistor; and a predictor circuit having a second digital input port,a second digital output port and an a second reset signal output port.12. The system of claim 11, wherein the second digital input port of thepredictor circuit is connected to a first digital output port of ananalog-to-digital converter.
 13. The system of claim 11, wherein thesecond reset signal output port of the predictor circuit is connected tothe first gate terminal of the reset transistor.
 14. The system of claim11, wherein the second digital output port of the predictor circuit isconnected to a third digital input port of a digital-to-analogconverter.
 15. The system of claim 14, wherein a third analog outputport of the digital-to-analog converter is connected to the second drainof the amplifier transistor.
 16. The system of claim 11, wherein ananode of the junction photodiode is connected to ground.